1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting problems with local area network (LAN) hardware. More particularly, the present invention relates to an apparatus and method for detecting noise on a LAN cable.
2. Related Art
A standard local area network (LAN) comprises a plurality of nodes connected to one another through a coaxial cable (called a LAN cable). The nodes communicate by sending packets of binary signals to one another. The binary signals comprise transitions between 0 and -2 volts.
A packet comprises a preamble, an address and the data to be communicated. The preamble is a sequence of -2 volt pulses to identify the beginning of the packet. The address is a number identifying the node to which the packet is being sent. The data is all or a portion of a information to be communicated between two nodes, for example.
The LAN cable can only convey one packet at a time. Therefore, before sending a packet, a node must determine that there are no packets currently being transmitted over the LAN cable. A node will make such a determination if the average voltage level on the LAN cable (called the DC bias) is lower than a particular voltage level (called the signal detect level). The signal detect level could be, for example, -0.3 volts.
The determination is based on the following. During transmission of the preamble, the sequence of -2 volt pulses brings the DC bias down to approximately one-half the peak voltage (-1 volt). Because this voltage is below the signal detect level, a node would detect a packet on the LAN cable.
During the transmission of addresses and data, the voltage on the LAN cable is likewise below the signal detect level. The binary representation of addresses and data is encoded so that there is a limited amount of time between transitions of 0 and -2 volts. The limited amount of time is too short for the DC bias to rise much above -1 volt.
FIG. 1A shows the level of a DC bias 102 during the first several pulses of a first preamble 104. The sequence of -2 volt pulses brings the DC bias 102 down to -1 volt. Because this is below the signal detect level, other nodes on the LAN are capable of determining that there is a packet being transmitted over the LAN cable. Therefore, generally, no other node will send a packet.
Occasionally, however, a node will send a packet while there is another packet on the LAN cable, causing what is called a collision. A collision occurs when two nodes send packets approximately simultaneously. To appropriately handle collisions, a node must continue to monitor the DC bias while it is transmitting. A node determines that a collision has occurred if the DC bias drops below a particular voltage level called the collision detect level. The latter is approximately -1.3 volts.
The determination is based on the following. If two nodes are simultaneously transmitting, they will bring the DC bias 102 to approximately twice what it would be if only one node were transmitting. Therefore, during a collision, the DC bias 102 is approximately -2 volts, which is below the collision detect level.
Upon detecting a collision, transmitting nodes continue to transmit for a short period of time to enable other nodes to detect the collision. Subsequently, all nodes temporarily cease transmitting.
FIG. 1A shows the level of the DC bias 102 during the first several pulses of second and third preambles 108 and 109 which collide. Looking at FIG. 1A, the second preamble 108 initially brings the DC bias 102 below the signal detect level. Shortly after the beginning of the second preamble 108, the third preamble 109 begins. The -2 volt pulses of the second preamble 108 overlap the -2 volt pulses of the third preamble 109 (as shown at 110), pulling the peak voltage to -4 volts. The overlapping preambles 110 bring the DC bias 102 down to one-half the peak voltage (-2 volts). This level is below the collision detect level, and a collision is therefore detected.
In FIG. 1A, the pulses of the second and third preambles 108 and 109 are approximately 90 degrees out of phase. Note that overlapping preambles would bring the DC bias 102 below the collision detect level regardless of how they overlapped. For example, if the pulses were 180 degrees out of phase, they would combine to place a steady -2 volt signal on the LAN cable and thereby bring the DC bias 102 to -2 volts. Alternatively, if the pulses were in phase with one another, they would combine to produce a square wave with a -4 volt peak. Again, the DC bias 102 would be -2 volts.
Noise on a LAN cable can substantially interfere with the transmission of packets. The most common source of noise on a LAN cable is commercial power resulting from two or more grounds on the LAN cable. A ground can be caused by, for example, an improperly insulated LAN cable or a faulty media attachment unit (MAU).
FIG. 1B shows a LAN 100 having a LAN cable 120 which is grounded in two places to thereby transmit commercial power. The LAN cable 120 comprises an center conductor 122 and an outer conductor 124. At each LAN cable end 126, the center conductor 122 is electrically connected to the outer conductor 124 through a 25 ohm resistor 128.
Each of two nodes 134 is attached to the LAN cable 120 through a faulty MAU 136. (The faults of the MAUs are depicted as ground connections off the MAUs.) Each MAU 136 is electrically connected to the LAN cable 120 through a "T" connection 138. Each MAU 136 is electrically connected to one of the nodes 134 through an Attachment Unit Interface (AUI) cable 140.
There is no noise on the portion of the LAN cable 120 between the MAUs 136. However, there is noise at 60 Hz on the portion of the LAN cable 120 between each MAU 136 and the nearest cable end 128.
FIG. 2 shows how noise from a commercial power source can interfere with data transmissions on the LAN 100 of FIG. 1B. A noise curve 202 represents noise from a commercial power source. The noise interferes with packet transmission because the DC bias is relative to the noise level. When a first packet 204 is sent, the noise curve 202 is at a noise positive peak. The DC bias 206 during transmission of the first packet 204 is above the signal detect level. As a result, the first packet 204 is not detected.
When a second packet 206 is sent, the noise curve 202 is at approximately 0 volts. The DC bias 208 during the second packet 206 is approximately -1 volt, which is between the signal detect level and the collision detect level. The second packet 206 therefore transmits normally.
During the time when the noise curve 202 is close to a noise negative peak, the noise curve 202, and thus the DC bias 210, are below the signal detect level. The nodes 134 interpret the noise as a packet and cease transmitting until the noise curve rises above the signal detect level.
When a third packet 212 is sent, the noise curve 202 is slightly above the signal detect level. The DC bias 214 during transmission of the third packet 212 falls below the collision detect level. The third packet 212 is therefore interpreted as a collision.
The problem of LAN noise from a source of commercial power is generally addressed by detecting its presence, locating its origin and eliminating it. Several well-known approaches are used to detect LAN noise. One such approach is by measuring the total power on the LAN cable 120. Because signals as well as noise generate power, this approach can only be used when no nodes 134 are transmitting. Accordingly, all nodes 134 must be turned off before the measurement is taken. Doing so would be undesirable on a busy network.
A second approach for detecting LAN noise would be to check for shorts to ground using an ohmmeter. But detection of a ground does not necessarily indicate the presence of LAN noise because LAN noise only occurs when the LAN cable 120 is grounded in more than one place.
A third approach is to search for certain collision patterns indicative of LAN noise. This is often impractical because such patterns are topologically dependent. Much domain-specific information is therefore required to detect them.
A fourth approach is to examine a LAN cable 120 with an oscilloscope. This approach is often impractical because of the high level of expertise required by the oscilloscope operator in order to distinguish noise from data. Also, oscilloscopes generally ground the LAN cable 120 during measurement. Thus, an oscilloscope would not be effective on a LAN cable 120 which was grounded exactly once. Furthermore, the limited portability of oscilloscopes makes the fourth approach difficult to physically carry out.
Therefore, what is needed is an easy to use apparatus or method for detecting LAN noise which requires neither disruption of the LAN nor substantial operator expertise.